One of the disadvantages of semiconductor RAMs (random access memories) is that they are volatile. These memory devices retain their information as long as power is supplied to them. However, as soon as power is lost, the stored information is also lost.
Different types of ROMs (read only memories) are used for information which is required to be stored in a nonvolatile memory. Recently EPROMs (erasable programmable read only memories) and EEPROMs (electrically erasable programmable read only memories) have become widely available. In an EPROM the user may program the device electrically and erase the entire device by exposing it to ultraviolet light. In an EEPROM the device is programmable and erasable electrically.
While these devices have the desired feature of nonvolatility, they are somewhat more inconvenient to use than a RAM. The EPROM and EEPROM require higher voltages to program (or WRITE) the device than the operating semiconductor RAMs. Furthermore, high voltages are required to erase information from the EEPROM. The EPROM requires removal of the device from the system for exposure to ultraviolet light. Thus these nonvolatile devices do not have the ease by which a RAM can store and change the stored information. Thus, many electronic memory systems have an organization in which RAMs store transient information, such as process data, and some kind of ROM to retain information which would be inconvenient to lose in case of a power outage. Such information might be the program to process the data. In any case, two sets of semiconductor devices are required.
The present invention provides for a semiconductor device which has the advantages of a RAM and an EEPROM. In one mode data can be stored and retrieved easily. In a second mode data can be stored in a nonvolatile state so that the danger of power outages is eliminated. These features are combined in one device so that the purchase of two different types of memory devices is eliminated.